Dendritic cells (DC) are among the first cells encountered by infecting pathogens; acting as sentries, these cells recognize invading microorganisms and secrete signals that dictate class differentiation of adaptive immunity. The mechanisms that mediate the ability of DC to discriminate between pathogens remain elusive. Using experimental Leishmania infection we will evaluate the precise contributions that host and parasite factors play in generating interleukin-12 (IL-12), the critical cytokine driving Thl cell differentiation. To model the initial events during Leishmania infection, we employ an in vitro system of human DC and infectious stage parasites. We previously demonstrated that the induction of IL-12 by these cells is Leishmania species dependent and requires CD40L stimulation. Whereas infection by species responsible for fatal visceral disease (L. donovani) fail to induce IL-12, infection with L. major a species associated with self-limiting cutaneous disease, primes DC for IL-12 secretion. The overall goal of this proposal is to identify the molecular determinants of these disparate IL-12 responses. Specific Aim 1 will determine the point of regulation at which Leishmania spp. modulate IL-12 production. A conditional approach will be taken to determine if transcriptional or post-transcriptional regulation plays a role in the production of IL-12 in response to Leishmania infection. Specific Aim 2 will identify the Leishmania factors that regulate IL-12 induction by specifically investigating the influence of different structural modifications on the parasite surface. Intrinsic differences in the ability of Leishmania parasites to prime DC for IL-12 production likely influences the capability of these parasites to activate Thl adaptive responses. These studies are underscored by the fact that the immune mechanisms elicited by live L. major infection leads to powerful life-long immunity. Elucidation of the critical components involved will have ramifications for other pathogen infections that require sustained cell-mediated responses for protection.